1. Technical Field
This disclosure relates to an airflow classifier to perform a classification operation on a particulate material, and to a particulate material preparing apparatus to prepare a particulate material having a desired particle size using the airflow classifier.
2. Discussion of the Related Art
There are particulate material preparing apparatuses including a pulverization and coarse particle classification device and a fine particle classification device. Conventional pulverization and coarse particle classification devices typically include two sets of one pulverizer and one classifier connected with the pulverizer, or a combination of one pulverizer and two classifiers. Specific examples of the pulverizer include jet mills in which a particulate raw material is fed into jet airflow spouted from a jet nozzle so that particles of the raw material collide against each other or a collision material (such as collision plates and walls), resulting in pulverization of the raw material; and mechanical pulverizers in which a particulate raw material is fed to a gap between a rotor having convexes and concaves on the surface thereof and a stator also having convexes and concaves so that the raw material is collided against the rotor and stator by swirling airflow caused by the rotor and stator. After the raw material is pulverized by one or two of such pulverizers, the pulverized material is classified with a combination of two coarse particle classifiers. The particulate material from which coarse particles are removed (i.e., relatively fine particles) are then subjected to fine particle classification using a fine particle classification device in which two classifiers are typically connected.
FIG. 1 is a schematic view illustrating a conventional particulate material preparing apparatus.
As illustrated on the left side of FIG. 1, a pulverization and coarse particle classification process is performed in a closed circuit. Specifically, a raw material is fed to a pulverizer 82 through a supply tube 81 to be pulverized. The pulverized raw material is collected once by a cyclone 84, and then fed to a coarse particle classifier 85 to be classified into relatively fine particles and relatively coarse particles to be pulverized again. The coarse particles are returned to the pulverizer 82 through a pipe 83 to be pulverized. The fine particles are fed to a cyclone 87 through a passage 86 to be collected.
In the coarse particle classification process, the particulate material fed to the classifier 85 is the pulverized raw material (i.e., the particulate material in the process of pulverization), and therefore the particulate material circulating in the circuit has a broad particle diameter distribution. When a product (particulate materials such as toner) having a desired particle diameter distribution is obtained from the particulate material, the yield of the product is very low, and the apparatus has to be operated with a heavy load because the amount of the particulate material returned to the pulverizer is large.
The fine particles collected by the cyclone 87 are subjected to a fine particle classification process. Specifically, the fine particles are then fed to a fine particle classifier 88 to be further classified, and particles having particle diameters in a desired particle diameter range are collected as a product in a product container 89.
The fine particles thus obtained in the fine particle classification process are collected by a cyclone 91 through a passage 90, and then fed to a fine particle classifier 94 through a passage 92 to be further classified. In this case, the relatively large particles are returned to the classifier 88 through a passage 93 while relatively fine particles are fed to a cyclone 96 through a passage 95 to be collected, and the collected fine particles are then fed to a container 97. A one-step or two-step classifier can be used for each of the fine particle classifiers 88 and 94 with consideration of the processing ability thereof.
In this system, a mixture of particles of the raw material and particles in process of pulverization, whose particle diameters fall in a wide particle diameter range, is circulated between a pulverizer and a classifier, and therefore the mixture has a very broad particle diameter distribution. Therefore, in order to prepare a product having a particle diameter in a desired particle diameter range, the system has to be operated with a heavy load. Accordingly, the resultant product tends to include a large amount of undesired coarse particles deteriorating image qualities. Meanwhile, relatively coarse particles returned to the pulverizer to be pulverized again include a large amount of relatively fine particles, which need not be further pulverized. Since such relatively fine particles are also pulverized by the pulverizer, the amount of finer particles included in the resultant pulverized particles increases, and in addition a problem in that the finer particles aggregate is also caused. Therefore, when the thus pulverized particles are classified in the following classification process to obtain a product, the yield of the product is low.
Since a toner prepared by such a conventional particulate material preparing apparatus includes large amounts of coarse particles and fine particles, the particles of the toner have a wide range of charge quantities, resulting in variation of the image density of images produced by the toner. In addition, excessively pulverized toner particles cause a background development problem in that the background area of an image is soiled with the toner particles, and coarse toner particles, which are not sufficiently pulverized, cause an insufficient transfer problem in that the toner particles are not transferred to an image area of an image bearing member, resulting in formation of an image having omissions. In addition, since a heavy load is applied on the classifiers of the particulate material preparing apparatus as mentioned above, the apparatus has low classification efficiency while having low pulverization energy efficiency in the pulverization process.
Recent image forming apparatuses typically form digital images using toner and are required to produce high quality images. Therefore, toner used for such image forming apparatuses is also required to have a sharp particle diameter distribution while minimizing the amounts of coarse and fine particles. Since conventional pulverizers consume much energy for a fine pulverization process, the pulverizers are not economically preferable for producing such toner. In addition, since jet air pulverizers produce toner including coarse particles in a relatively large amount of from 1% to 15% by weight, it is necessary to remove such coarse particles, resulting in deterioration of production efficiency. Alternatively, it is necessary to further perform a pulverization operation. In this case, the energy consumption is increased. Further, since the above-mentioned conventional pulverization/classification apparatuses have insufficient pulverization properties such as pulverization processing ability and energy consumption, the toner produced by the apparatuses has broad particle diameter distribution and broad charge quantity distribution, and therefore it is difficult for the toner to produce high quality images.
In attempting to efficiently produce a particulate material having a sharp particle diameter distribution while minimizing the amount of fine particles and coarse particles, airflow classifiers including a dispersing chamber and a classification chamber have been proposed. However, it is necessary for such airflow classifiers to improve the coarse particle classification accuracy.
Because of these reasons, the inventor recognized that there is a need for a particulate material preparing apparatus which can produce a particulate material having a desired particle diameter distribution (such as volume average particle diameter of not greater than 5 μm) while minimizing the amount of fine particles and coarse particles.